Cast acrylic plates with enhanced shockproofness

ABSTRACT

Process for the preparation of impact-strengthened cast PMMA sheets comprising the following stages:
     1. a mixture comprising:
       at least one alkoxyamine Z(-T) n , in which Z denotes a polyvalent group and n an integer greater than 2, preferably of between 2 and 10, advantageously between 2 and 8, and   the monomer(s) intended to form a central block B   
        is heated to a temperature sufficient to activate the alkoxyamine and polymerize the monomer(s);   2. the central block B, optionally mixed with the unconsumed monomer(s) from stage 1, is reactivated in the presence of the monomer(s) intended to form the branches A;   3. MAM and optionally at least one comonomer M which can be copolymerized by the radical route with MMA and at least one radical initiator are added to the mixture obtained in stage 2;   4. the mixture from stage 3 is cast in a mould and then heated.

The present invention relates a process for the manufacture ofimpact-strengthened cast acrylic sheets. It also relates to cast sheetsper se and to their uses.

Polymethyl methacrylate (PMMA) is a material valued for its excellentoptical properties (in particular the gloss and a high transparency witha transmission of visible light of at least 90%). However, it is also abrittle thermoplastic material which is sensitive to impacts. Thischaracteristic is related to the fact that the glass transitiontemperature of PMMA is approximately 110° C., so that, in this material,the polymer chains are not capable of easily moving at ambienttemperature. For some applications, it is therefore necessary to improvethe impact strengthening of PMMA while retaining its transparency.

The impact strengthening of PMMA is generally improved by virtue of theintroduction into the acrylic resin of an impact modifier of the typewhich is known as core-shell, which exists in the form of multilayerspherical particles. These particles are prepared by emulsionpolymerization and are recovered in the powder form by atomization. Theycomprise a sequence of “hard” and “soft” layers. It is thus possible toencounter bilayer (soft-hard) or trilayer (hard-soft-hard) particles. Inthe case of cast acrylic sheets, obtained by polymerization of themixture of monomers in a mould, the impact modifier is dispersedbeforehand in the mixture of monomers. In the case of extruded acrylicsheets, the impact modifier is compounded in the extruder with theacrylic resin. In both cases, it is necessary for the impact modifier tobe well dispersed in the acrylic resin in order to maintain a constantand homogeneous level of impact strength.

PRIOR ART

Application WO 01/57133 of the Applicant Company discloses a methacrylic(co)polymer strengthened by an impact modifier and by a graftedelastomeric copolymer. The impact modifier can be an additive ofcore-shell type or a block copolymer comprising at least one blockobtained from a diene or from an alkyl or aralkyl (meth)acrylate. Thegrafted elastomeric copolymer is obtained from an elastomeric copolymerto which methacrylic (co)polymer groups are grafted in the pendentposition. The impact strengthening thus results from the combination oftwo polymers, the impact modifier and the grafted elastomeric copolymer.

International Application WO 99/29772 discloses the impact strengtheningof semi-crystalline thermoplastic resins using astyrene-butadiene-methyl methacrylate block copolymer (SBM).

International Application WO 02/055573 of the Applicant Companydiscloses the impact strengthening of a PMMA using a block copolymer ofABA type in which B denotes a central block obtained from a diene, forexample an SBM.

International Application Wo 03/062293 of the Applicant Companydiscloses a process for the impact strengthening of a thermoplasticmatrix using a B(-A)_(n) block copolymer composed of a central block Band of n branches A and prepared using the controlled radicalpolymerization technique. This process applies to the strengthening ofnumerous thermoplastics (PS, PC, PVDF, and the like) and in particularto the manufacture of cast PMMA sheets.

According to this process, in a 1st stage, the central block B isprepared using a polyfunctional alkoxyamine. In a 2nd stage, the centralblock B is mixed with the monomer(s) intended to form the branches A,which results in the formation of the B(-A)_(n) block copolymer. In thisstage, a radical initiator can be added to the mixture, which results inthe formation of a matrix. In a 3rd stage, the block copolymerB(-A)_(n), optionally mixed with the matrix, is separated from theresidual monomers by evaporation under vacuum at temperatures ranging upto 250° C. (stage referred to as desolventization stage). In a 4thstage, the block copolymer, thus freed from the residual monomers, cansubsequently be extruded with a thermoplastic resin or else redissolvedin a mixture of monomers, which is itself subsequently polymerized. Onconclusion of this 4th stage, a block copolymer dispersed in a matrix isthus obtained.

The process of WO 03/062293, applied to the manufacture of cast sheets,is not capable of transfer to the industrial scale. This is because itexhibits the disadvantage of requiring a stage of desolventization,followed by a stage of redissolution of the copolymer. First, these twounit operations, by increasing the overall cycle time, affect the outputof the process. Secondly, the desolventization stage is also capable ofresulting in the formation of gels in the block copolymer, which affectsits redissolution in the mixture of monomers and, consequently, candamage the transparency of the cast sheet.

Furthermore, according to the process disclosed, in particular in theexamples, it is preferable, during the 2nd stage, to initiate theformation of the branches A at the same time as that of the matrix. Forthis, the monomer A is brought into contact with two types ofinitiators, the conventional radical initiator and the reactivatablecentral block. The monomer A is thus consumed at the same time accordingto two competing radical polymerization mechanisms, each exhibitingspecific kinetics. The control of this 2nd stage is very difficult as itimplies the harmonization of the rates of formation of the blocks A andof the matrix. This implies that it is necessary to adjust the nature ofthe radical initiator to the central block B and thus also to carefullyadjust the temperature cycle. In practice, contradictory requirementsare encountered and the possible compromises generally result:

-   -   in premature phase separation during the polymerization of the        copolymer B(-A)_(n), which migrates to the interface of the        sheet and mould. In this case, sheets which are impossible to        remove from the mould and/or which are partially or completely        opaque are obtained;    -   in unacceptable contents of residual methyl methacrylate (MMA),        which it is impossible to remove once the sheet is finished.

The Applicant Company has now improved the process for the preparationof impact-strengthened cast acrylic sheets disclosed in InternationalApplication WO 03/062293. The cycle time of this process is improvedwith respect to that disclosed in WO 03/062293 as it does not requireany desolventization-redissolution stage. The process of the inventionthus exhibits an improved productive output.

Furthermore, the radical initiator is added after and not during theformation of the block copolymer B(-A)_(n), which facilitates thecontrol of the polymerization and, consequently, makes it possible toavoid the formation of defects at the surface of the sheet, theformation of opaque regions due to the phase separation of the copolymerB(-A)_(n) and the presence of an unacceptable amount of residual MMA.

The Applicant Company has also found, with surprise, that a very goodtransparency/impact strength compromise is obtained if the proportion ofthe central block B in the sheet is between 2 and 5%, preferably between2.5 and 4.5%, more advantageously still between 2.6 and 4.0%.

According to an alternative form of the invention, the manufacture ofcast sheets can also be envisaged starting from a block copolymerB(-A)_(n) preformed elsewhere. For example, it is possible to envisage,for example for reasons of cost or logistics, preparing the copolymer ona production site other than that of the cast sheets, optionally even byanother manufacturer. The process then comprises the following stages:

-   1. mixing the block copolymer B(-A)_(n) with MMA and optionally with    at least one comonomer M and with at least one radical initiator;-   2. casting the mixture obtained in stage 1 in a mould and then    heating it in order to obtain a cast sheet.

Preferably, the proportion by weight of the central block B in the sheetis between 2 and 5%, preferably between 2.5 and 4.5%, moreadvantageously still between 2.6 and 4.0%.

Furthermore, the block copolymer has a tendency to settle down insidethe matrix to give homogeneously distributed particles. The particlesexist in the form of substantially spherical nodules inside which one ormore subnodule(s), having the same composition as the MMA homo- orcopolymer, are present.

BRIEF DESCRIPTION OF THE INVENTION

A first subject-matter of the invention is a process for the preparationof cast sheets made of PMMA strengthened with regard to impactcomprising the following stages:

-   1. a mixture comprising:    -   at least one alkoxyamine Z(-T)_(n), in which Z denotes a        polyvalent group and n an integer greater than 2, preferably of        between 2 and 10, advantageously between 2 and 8, and    -   the monomer(s) intended to form a central block B-    is heated to a temperature sufficient to activate the alkoxyamine    and polymerize the monomer(s);-   2. the central block B, optionally mixed with the unconsumed    monomer(s) from stage 1, is reactivated in the presence of the    monomer(s) intended to form the branches A;-   3. MAM and optionally at least one comonomer M which can be    copolymerized by the radical route with MMA and at least one radical    initiator are added to the mixture obtained in stage 2;-   4. the mixture from stage 3 is cast in a mould and then heated.

According to an alternative form of the invention, the process comprisesthe following stages:

-   -   1. a mixture is prepared comprising:    -   a block copolymer B(-A)_(n) composed of n branches A as defined        in either of claims 4 and 5 and connected to their covalent        bonds to a central block B as defined in either of claims 2 and        3, n denoting an integer of greater than or equal to 2,        preferably between 2 and 10 and advantageously between 2 and 8    -   MAM and    -   optionally at least one comonomer M and at least one radical        initiator;

-   2. the mixture obtained in stage 1 is cast in a mould and then it is    heated in order to obtain a cast sheet.

Preferably, the content of central block B in the sheet is between 2 and5%, preferably between 2.5 and 4.5%, more advantageously still between2.6 and 4.0%.

The invention also relates to a sheet capable of being obtainedaccording to the process or its alternative form.

The invention also relates to an MMA home- or copolymer in whichparticles of block copolymer B(-A)_(n), existing the form ofsubstantially spherical nodules inside which one or more subnodule(s)having the same composition as the MMA home- or copolymer are present,are homogeneously dispersed. It also relates to a cast sheet comprisingsuch a homo- or copolymer. Finally, it relates to the use of the castsheet in the manufacture of window panes, soundproof walls, flatscreens, billboard or display devices.

DETAILED DESCRIPTION

As regards the block copolymer B(-A)_(n), the latter is composed of nbranches A connected via covalent bonds to a central block B, n denotingan integer greater than or equal to 2, preferably between 2 and 10 andadvantageously between 2 and 8. The branches A can be identical ordifferent, that is to say have identical or different average molecularweights and/or compositions.

According to the definition given by the IUPAC (see IUPAC Compendium ofChemical Terminology, 2nd edition (1997), 1996, 68, 2303), a blockcopolymer is composed of macromolecules having several chemicallydifferent polymer blocks, that is say derived from different monomers oralso derived from the same monomers according to differentdistributions, which are connected to one another via covalent bonds.Reference may also be made to Kirk-Othmer Encyclopaedia of ChemicalTechnology, 3rd ed., vol. 6, p. 798, for further details with regard tocopolymers this type. The block copolymer can be linear, star or comb(brush copolymer). Preferably, it is a linear copolymer, more preferablystill a linear triblock copolymer of formula ABA (n=2). The blockcopolymer B(-A)_(n) is different from the particles of core-shell type.

In the context of the invention, the copolymer can be a triblockcopolymer with, in this case, n=2 (a central block and 2 branches).Examples of triblock copolymers can be PMMA-b-poly(n-butylacrylate)-b-PMMA, PMMA-b-poly(n-butyl acrylate-co-styrene)-b-PMMA,PMMA-b-poly(isobutyl acrylate-co-styrene)-b-PMMA, poly(MMA-co-n-butylacrylate)-b-poly(n-butyl acrylate-co-styrene)-b-poly(MMA-co-n-butylacrylate) (b: symbol used to denote a block copolymer, co: symbol usedto denote a random copolymer).

The block copolymer B(-A)_(n) is prepared by the controlled radicalpolymerization technique using an alkoxyamine of formula Z(-T)_(n). Bythis technique, the branches A are terminated by the nitroxide T, all orelse in part. The branches can be terminated in part by the nitroxide Twhen, for example, a transfer reaction occurs between a nitroxide and analkyl methacrylate, as indicated in the reaction below:

The block copolymer B(-A)_(n) exhibits a weight-average molecular massof between 30 000 and 300 000 g/mol, preferably between 35 000 and 300000 g/mol. The polydispersity index is between 1.5 and 3.0, preferablybetween 1.8 and 2.7 and more preferably between 1.9 and 2.7.

As regards the central block B, the latter exhibits an overall glasstransition temperature (recorded as T_(g)) of less than 0° C.Preferably, its weight-average mass is greater than 5000 g/mol,preferably greater than 20 000 g/mol and advantageously greater than 30000 g/mol. It is preferably between 30 000 and 300 000 g/mol,advantageously between 50 000 and 250 000 g/mol.

The central block B is prepared from a mixture comprising at least onemonomer chosen from:

-   -   acrylic monomers of formula CH₂═CH—C(═O)—O—R₁ where R₁ denotes a        hydrogen atom or a linear, cyclic or branched C₁-C₄₀ alkyl group        optionally substituted by a halogen atom or a hydroxyl, alkoxy,        cyano, amino or epoxy group, such as, for example, acrylic acid,        methyl, ethyl, propyl, n-butyl, isobutyl, tert-butyl,        2-ethylhexyl or glycidyl acrylate, hydroxy-alkyl acrylates or        acrylonitrile;    -   methacrylic monomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ where R₂        denotes a hydrogen atom or a linear, cyclic or branched C₁-C₄₀        alkyl group optionally substituted by a halogen atom or a        hydroxyl, alkoxy, cyano, amino or epoxy group, such as, for        example, methacrylic acid, MMA, ethyl, propyl, n-butyl,        isobutyl, tert-butyl, 2-ethylhexyl or glycidyl methacrylate,        hydroxyalkyl methacrylates or methacrylonitrile;

vinylaromatic monomers, such as, for example, styrene, substitutedstyrenes, α-methylstyrene, monochlorostyrene or tert-butylstyrene.

It is not prepared from a diene. A person skilled in the art knows howto combine these monomers so as to regulate the overall T_(g) of thecentral block B. In order to obtain a T_(g) of less than 0° C., it isnecessary to use at least one monomer exhibiting a T_(g) of less than 0°C., for example butyl acrylate or 2-ethylhexyl acrylate. The refractiveindex of the central block B is preferably as close as possible to thatof the matrix in order to provide the best possible transparency.

The central block B can be composed solely of a monomer exhibiting aT_(g) of less than 0° C., for example butyl acrylate or 2-ethylhexylacrylate. The central block B can also be composed of at least one alkylacrylate and of a vinylaromatic monomer. Advantageously, it is composedof butyl acrylate and of styrene in the butyl acrylate/styrene ratio byweight of between 70/30 and 90/10, preferably between 75/25 and 85/15.

As regards the branches A, the latter exhibit an overall T_(g) ofgreater than 0° C. and are compatible with the MMA home- or copolymer.

Preferably, the weight-average mass of each branch A is between 15 000and 1 000 000 g/mol, preferably between 25 000 and 500 000 g/mol,advantageously between 35 000 and 300 000 g/mol. Preferably, in order toimprove the compatibility of the block copolymer B(-A)_(n) with themethacrylic (co)polymer and the transparency of the sheet, the ratio byweight of the branches A to the central block B is greater than 1,preferably greater than 1.5.

Each branch A is prepared from a mixture comprising MMA and optionallyat least one monomer chosen from:

-   -   acrylic monomers of formula CH₂═CH—C(═O)—O—R₁ where R₁ denotes a        hydrogen atom or a linear, cyclic or branched C₁-C₄₀ alkyl group        optionally substituted by a halogen atom or a hydroxyl, alkoxy,        cyano, amino or epoxy group, such as, for example, acrylic acid,        methyl, ethyl, propyl, n-butyl, isobutyl, tert-butyl,        2-ethylhexyl or glycidyl acrylate, hydroxy-alkyl acrylates or        acrylonitrile;    -   methacrylic monomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ where R₂        denotes a hydrogen atom or a linear, cyclic or branched C₂-C₄₀        alkyl group optionally substituted by a halogen atom or a        hydroxyl, alkoxy, cyano, amino or epoxy group, such as, for        example, methacrylic acid, ethyl, propyl, n-butyl, isobutyl,        tert-butyl, 2-ethylhexyl or glycidyl methacrylate, hydroxyalkyl        methacrylates or methacrylonitrile;    -   vinylaromatic monomers, such as, for example, styrene or        substituted styrenes, such as α-methylstyrene, monochlorostyrene        or tert-butylstyrene.

MMA predominates. Preferably, each branch A includes a proportion byweight of MMA of between 50 and 100%, preferably between 75 and 100%,advantageously between 90 and 100%.

The optional comonomer M is any comonomer which can be copolymerized bythe radical route with MMA. It is preferably chosen from:

-   -   acrylic monomers of formula CH₂═CH—C(═O)—O—R₁ where R₁ denotes a        hydrogen atom or a linear, cyclic or branched C₁-C₄₀ alkyl group        optionally substituted by a halogen atom or a hydroxyl, alkoxy,        cyano, amino or epoxy group, such as, for example, acrylic acid,        methyl, ethyl, propyl, n-butyl, isobutyl, tert-butyl,        2-ethylhexyl or glycidyl acrylate, hydroxyalkyl acrylates or        acrylonitrile;    -   methacrylic monomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ where R₂        denotes a hydrogen atom or a linear, cyclic or branched C₂-C₄₀        alkyl group optionally substituted by a halogen atom or a        hydroxyl, alkoxy, cyano, amino or epoxy group, such as, for        example, methacrylic acid, ethyl, propyl, n-butyl, isobutyl,        tert-butyl, 2-ethylhexyl or glycidyl, hydroxyalkyl methacrylates        or methacrylonitrile;    -   vinylaromatic monomers, such as, for example, styrene,        substituted styrenes, α-methylstyrene, monochlorostyrene or        tert-butylstyrene;    -   polyfunctional acrylic monomers which give rise to crosslinking,        such as, for example, polyol polyacrylates, alkylene glycol        polyacrylates or allyl acrylate, ethylene glycol diacrylate,        1,3-butylene glycol diacrylate or 1,4-butylene glycol        diacrylate;    -   polyfunctional methacrylic monomers which give rise to        crosslinking, such as polyol polymethacrylates, alkylene glycol        polymethacrylates or allyl methacrylate, ethylene glycol        dimethacrylate, 1,3-butylene glycol dimethacrylate or        1,4-butylene glycol dimethacrylate;    -   vinylaromatic monomers, such as, for example, styrene,        substituted styrenes, α-methylstyrene, monochlorostyrene or        tert-butylstyrene;    -   polyfunctional vinylaromatic monomers which give rise to        crosslinking, such as, for example, divinylbenzene or        trivinylbenzene.

Preferably, M is methyl acrylate, ethyl acrylate, butyl acrylate or1,4-butanediol dimethacrylate (BDMA). The MMA/comonomer M proportion byweight is between 90/10 and 100/0, preferably between 95/5 and 100/0.

As regards the alkoxyamine, the latter has the general formula Z(-T)_(n)in which Z denotes a polyvalent group capable of relasing severalradical sites after activation. The activation occurs by cleavage of theZ-T covalent bonds. n is an integer greater than 2, preferably ofbetween 2 and 10, advantageously between 2 and 8.

n represents the functionality of the alkoxyamine, that is to say thenumber of nitroxides T which can be released by the alkoxyamineaccording to the mechanism:

In the presence of monomer(s), the alkoxyamine activated by heatinginitiates the polymerization. The preparation of a block copolymerpolyM2-polyM1-polyM2 from an alkoxyamine for which n=2 is illustrated inthe scheme below. The monomer M1 is first polymerized after activationof the alkoxyamine and then the finished polyM1 block is reactivated toinitiate the polymerization of the monomer M2:

The principle of the preparation of block copolymers remains valid forn>2.

By way of example, Z can be chosen from the following groups (I) to(VIII):

in which R₃ and R₄, which are identical or different, represent a linearor branched alkyl radical having a number of carbon atoms ranging from 1to 10, phenyl or thienyl radicals optionally substituted by a halogenatom, such as F, Cl or Br, or else by a linear or branched alkyl radicalhaving a number of carbon atoms ranging from 1 to 4 or else by nitro,alkoxy, aryloxy, carbonyl or carboxyl radicals; a benzyl radical, acycloalkyl radical having a number of carbon atoms ranging from 3 to 12,a radical comprising one or more unsaturations; B represents a linear orbranched alkylene radical having a number of carbon atoms ranging from 1to 20; m is an integer ranging from 1 to 10;

in which R₅ and R₆, which are identical or different, represent aryl,pyridyl, furyl or thienyl radicals optionally substituted by a halogenatom, such as F, Cl or Br, or else by a linear or branched alkyl radicalhaving a number of carbon atoms ranging from 1 to 4 or else by nitro,alkoxy, aryloxy, carbonyl or carboxyl radicals; D represents a linear orbranched alkylene radical having a number of carbon atoms ranging from 1to 6, a phenylene radical, a cycloalkylene radical; p being an integerranging from 1 to 10;

in which R₇, R₈ and R₉, which are identical or different, have the samemeanings as R₃ and R₄ of the formula (I) and q, r and s are integersranging from 1 to 10;

in which R₁₀ has the same meaning as R₅ and R₆ of the formula (II), t isan integer ranging from 1 to 4 and u is an integer of between 2 and 6(the aromatic group is substituted);

in which R₁₁ has the same meaning as the R₁₀ radical of the formula (IV)and v is an integer between 2 and 6;

in which R₁₂, R₁₃ and R₁₄, which are identical or different, represent aphenyl radical optionally substituted by a halogen atom, such as Cl orBr, or else by a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 10, W represents an oxygen, sulphur orselenium atom and w is equal to zero or 1;

in which R₁₅ has the same meaning as R₃ of the formula (I) and R₁₆ hasthe same meaning as R₅ or R₆ of the formula (II);

in which R₁₇ and R₁₆, which are identical or different, represent ahydrogen atom, a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 10, an aryl radical, optionallysubstituted by a halogen atom or a heteroatom.

T is a nitroxide, that is to say a stable free radical exhibiting an═N—O^() group on which a lone electron is present. The term “stablefree radical” denotes a radical so persistent and unreactive withrespect to the air and the moisture in the surrounding air that it canbe handled and stored for a much longer period of time than the majorityof free radicals (see in this respect, Accounts of Chemical Research,1976, 9, 13-19). It is thus distinguished from free radicals having afleeting lifetime (from a few milliseconds to a few seconds), such asthe free radicals resulting from standard polymerization initiators(peroxides, hydroperoxides or azo initiators). The free radicals whichare polymerization initiators tend to accelerate the polymerization,whereas the stable free radicals generally tend to slow it down. It maybe said that a free radical is stable within the meaning of the presentinvention if it is not a polymerization initiator and if, under thenormal conditions of the invention, the mean lifetime of the radical isat least one minute.

T is represented by the structure:

in which R₁₉, R₂₀, R₂₁, R₂₂, R₂₄ and R₂₄ denote:

linear or branched C₁-C₂₀, preferably C₁-C₁₀, alkyl groups, such asmethyl, ethyl, propyl, butyl, isopropyl, isobutyl, tert-butyl orneopentyl, which may or may not be substituted,

-   -   substituted or unsubstituted C₆-C₃₀ aryl groups, such as benzyl,        aryl(phenyl),    -   saturated C₁-C₃₀ cyclic groups,        and in which the R₁₉ and R₂₂ groups can form part of an        optionally substituted cyclic structure R₁₉—CNC—R₂₂ which can be        chosen from:

in which x denotes an integer between 1 and 12.

Use may be made, by way of examples, of the nitroxides:

In a particularly preferred way, the nitroxides of formula (X) are usedin the context of the invention since they allow the polymerization tobe well controlled:

R_(a) and R_(b) denote identical or different alkyl groups having from 1to 40 carbon atoms which are optionally connected to one another so asto form a ring and which are optionally substituted by hydroxyl, alkoxyor amino groups, R_(L) denotes a monovalent group with a molar mass ofgreater than 16 g/mol, preferably of greater than 30 g/mol. The R_(L)group preferably has a molar mass of between 40 and 450 g/mol. It ispreferably a phosphorus group of general formula (XI):

in which X and Y, which can be identical or different, can be chosenfrom alkyl, cycloalkyl, alkoxy, aryloxy, aryl, aralkyloxy,perfluoroalkyl or aralkyl radicals and can comprise from 1 to 20 carbonatoms; X and/or Y can also be a halogen atom, such as a chlorine,bromine or fluorine atom.

Advantageously, R_(L) is a phosphonate group of formula:

in which R_(c), and R_(d) are two identical or different alkyl groups,optionally connected so as to form a ring, comprising from 1 to 40carbon atoms, and optionally substituted or not.

The R_(L) group can also comprise at least one aromatic ring, such asthe phenyl radical or the naphthyl radical, substituted, for example, byone or more alkyl radical(s) comprising from 1 to 10 carbon atoms.

The alkoxyamines of formula (XIII) comprising the nitroxide of formula(X) are preferred:

Mention may be made, as examples of nitroxides of formula (X) which canbe carried by the alkoxyamine (XIII), ofN-tert-butyl-1-phenyl-2-methylpropyl nitroxide,N-(2-hydroxymethylpropyl)-1-phenyl-2-methylpropyl nitroxide,N-tert-butyl-1-dibenzylphosphono-2,2-dimethylpropyl nitroxide,N-tert-butyl-1-di(2,2,2-trifluoroethyl)phosphono-2,2-dimethyl-propylnitroxide, N-tert-butyl-[(1-diethylphosphono)-2-methylpropyl]nitroxide,N-(1-methylethyl)-1-cyclohexyl-1-(diethylphosphono) nitroxide,N-(1-phenylbenzyl)-[(1-diethylphosphono)-1-methylethyl]nitroxide,N-phenyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide,N-phenyl-1-diethylphosphono-1-methylethyl nitroxide,N-(1-phenyl-2-methylpropyl)-1-diethylphosphonomethylethyl nitroxide, orthe nitroxide of formula

The nitroxide of formula (XIV) is particularly preferred:

It is N-tert-butyl-1-diethylphosphono-2,2-dimethyl-propyl nitroxide,commonly referred to as SG1 for simplicity.

An alkoxyamine can be prepared by one of the procedures described, forexample, in US 590 549 or in FR 99.04405. One method which can be usedconsists in carrying out the coupling of a carbon radical with anitroxide. The coupling can be carried out starting from a halogenatedderivative in the presence of an organometallic system, such asCuX/ligand (X═Cl or Br), according to a reaction of ATRA (Atom TransferRadical Addition) type, as described by D. Greszta et al. inMacromolecules, 1996, 29, 7661-7670.

Alkoxyamines which can be used in the context of the invention arerepresented below:

G denotes the —P(═O)(OEt)₂ group.

It would not be departing from the scope of the present invention tocombine several alkoxyamines. These mixtures might thus comprise, forexample, an alkoxyamine having n1 attached nitroxides and an alkoxyaminehaving n2 attached nitroxides, with n1 different from n2. Thecombination might also be a combination of alkoxyamines carryingdifferent nitroxides.

As regards the radical initiator, they can be chosen from diacylperoxides, peroxy esters, dialkyl peroxides, peroxyacetals or azocompounds. The radical initiators which may be suitable are, forexample, isopropyl carbonate, benzoyl peroxide, lauroyl peroxide,caproyl peroxide, dicumyl peroxide, tert-butyl perbenzoate, tert-butylper(2-ethylhexanoate), cumyl hydroperoxide,1,1-di(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butylperoxyisobutyrate, tert-butyl peracetate, tert-butyl perpivalate, amylperpivalate, tert-butyl peroctoate, azobis-isobutyronitrile (AIBN),azobisisobutyramide, 2,2′-azo-bis(2,4-dimethylvaleronitrile) or4,4′-azobis(4-cyanopentancic. It would not be departing from the scopeof the invention to use a mixture of radical initiators chosen from theabove list. The preferred radical initiator is azobisisobutyronitrile.

The content of radical initiator with respect to the monomers of themixture which is cast in the mould varies from 100 to 2000 ppm (byweight), preferably between 200 and 1000 ppm, by weight. This contentcan vary according to the application targeted and the thicknesstargeted.

Other ingredients can optionally be added to the mixture which is castin the mould (during stage 3 of the process according to the inventionor stage 1 of the alternative form). Mention may be made, withoutimplied limitation, of organic dyes or inorganic pigments; plasticizers;UV stabilizers, such as Tinuvin® P from Ciba, which is used at contentsof 0 to 1000 ppm and preferably 50 to 500 ppm with respect to themixture which is cast in the mould; light or heat stabilizers, such as,for example, Tinuvin® 770; antioxidants; flame retardants, such as, forexample, tris(2-chloropropyl)phosphate; thickeners, such as, forexample, cellulose acetate butyrate; mould-release agents, such as, forexample, sodium dioctyl sulphosuccinate, used at contents of 0 to 500ppm and preferably 0 to 200 ppm with respect to the mixture which iscast in the mould; inorganic or organic fillers (for example, polyamide,PTFE, BaSO₄) intended to scatter light (for example, to give sheetswhich can be edge-lit) or to opacify the sheet. These fillers aregenerally used in the form of premanufactured pastes in a plasticizer ofdialkyl phthalate type; at least one additive having the role ofreflecting infrared radiation and/or one additive having the role ofblocking UV radiation.

The other ingredient can also be a chain-limiting agent commonly used inthe field of cast sheets, for example γ-terpinene or terpinolene, atcontents of between 0 and 500 ppm and preferably between 0 and 100 ppm,with respect to the monomers of the mixture which is cast in the mould.The chain-limiting agent can also be added before the formation of thebranches A (during stage 2 of the process according to the invention) atcontents of between 0 and 500 ppm and preferably between 0 and 100 ppm,with respect to the monomer(s) intended to form the branches A.

The impact strengthening according to the invention is produced byvirtue of the copolymer B(-A)_(n) but the addition in synergy of animpact modifier, for example of the core-shell type, is not ruled out,in particular of the soft-hard or hard-soft-hard type (for example soldunder the Durastrength® or Metablend® (for example, D320) trade marks ofArkema). The impact modifier/block copolymer B(-A)_(n) proportion canthus be between 90/10 and 10/90. Preferably, the copolymer B(-A)_(n) isused alone and no additive of core-shell type is added.

As regards the process, the latter comprises the following stages:

During the 1st stage, a mixture comprising at least one alkoxyamineZ(-T)_(n) and the monomer(s) intended to form the central block B isheated to a temperature sufficient to activate the alkoxyamine and topolymerize the monomer(s).

The temperature is chosen so as to activate the alkoxyamine whileretaining the living character of the controlled radical polymerization.Preferably, it is between 80 and 150° C., advantageously between 80 and130° C. This temperature is specific to the alkoxyamine used and to themonomer(s) to be polymerized. The duration of the polymerization canvary between 30 minutes and 8 hours, preferably between 1 and 8 hours,advantageously between 2 and 6 hours.

It is possible to also add nitroxide to the mixture in order to providebetter control of the polymerization. The nitroxide which is added maybe identical to that which is carried on the alkoxyamine or different.The molar proportion of the nitroxide added with respect to thealkoxyamine is between 0 and 20%, preferably between 0 and 10%.

The conversion of the monomer(s) can vary between 10 and 100%. However,preferably, the polymerization is halted for a conversion of between 50and 100% and advantageously between 50 and 80%.

On conclusion of this 1st stage, the central block B, optionally mixedwith the unconsumed monomer(s), is obtained.

During the 2nd stage, the central block B, optionally mixed with theunconsumed monomer(s), from the 1st stage is reactivated in the presenceof the monomer(s) intended to form the branches A.

If the conversion of the 1st stage is less than 100%, the not completelypolymerized monomer(s) from the 1st stage may be found in the mixture.The mixture thus comprises the central block B, the monomer(s) intendedto form the branches A which has/have been added and possibly themonomer(s) not completely polymerized in the 1st stage. The proportionof central block B in this mixture is between 1 and 20%, preferablybetween 1 and 15% and advantageously between 2 and 10% by weight.

The branches A are formed at a temperature of between 80 and 150° C.,preferably between 80 and 130° C. The duration of the polymerization canvary between 30 minutes and 8 hours, preferably between 1 and 4 hours,advantageously between 1 and 2 hours. As during stage 1, it ispreferable to avoid the presence of oxygen. It is possible to addnitroxide during this stage, it being possible for this nitroxide to bedifferent from that carried by the alkoxyamine. The proportion ofnitroxide added at this stage is between 0 and 20 mol %, preferablybetween 0 and 10 mol %.

During the 2nd stage, the conversion can vary between 10 and 100%.However, in order not to obtain an excessively viscous mixture, it ispreferable to limit the conversion to between 5 and 50%, preferablybetween 5 and 30%, so that the mixture obtained on conclusion of this2nd stage comprises the block copolymer B(-A)_(n) mixed with theunconverted monomer(s). This mixture is commonly referred to as “syrup”.

During the 3rd stage, MMA and optionally at least one other monomer Mand at least one radical initiator are added to the mixture obtained inthe 2nd stage.

On conclusion of the 3rd stage, a mixture comprising MMA, the copolymerB(-A)_(n), at least one radical initiator and optionally at least onecomonomer M is obtained. It may be that monomers unconsumed in the 1stand/or 2nd stages remain, along with possible chain-limiting agents. MMAis predominant.

Stages 1-3 can be carried out in a reactor which can be closed or open.It can be the same reactor for the 3 stages. It is preferable to avoidthe presence of oxygen. To do this, the reaction mixture is generallydegassed under reduced pressure and the reactor is rendered inert bybeing flushed with nitrogen or with argon after introducing thereactants.

During the 4th stage, the mixture from the 3rd stage is cast in a mouldand then heated. The mould is formed of two glass sheets separated by aseal made of PVC, for example. The heating can, for example, consist inusing a vessel filled with water or a ventilated oven in which themoulds with their mixture are placed in a row and which has atemperature which is modified.

As regards the alternative form of the process, the latter comprises thefollowing stages:

During the 1st stage, a block copolymer B(-A)_(n) is mixed with MMA andwith optionally at least one comonomer M and at least one radicalinitiator.

The block copolymer can preferably be reactivated (that is to say that,when it is heated in the presence of monomer(s), it initiates thepolymerization of the monomer(s). Preferably, the proportion of centralblock B in this mixture is, by weight, between 2 and 5%, preferablybetween 2.5 and 4.5%, more advantageously still between 2.6 and 4.0%.

The mixture comprises MMA, the copolymer B(-A)_(n), at least one radicalinitiator and optionally at least one comonomer M. The block copolymerB(-A)_(n) is said to be reactivatable.

During the 2nd stage, the mixture obtained in the 1st stage is cast in amould and then it is heated in order to obtain a cast sheet.

According to the invention or its alternative form, the heating of themixture cast in the mould can be carried out at a constant temperature(isotherm) or else it can follow a very precise temperature programme,for example a first stationary phase at approximately 70° C., followedby a second stationary phase at about 120° C. After cooling, the sheetobtained is removed from the mould.

The process of the present invention is applicable to the production ofindustrial acrylic sheets of various thicknesses, advantageously between2 and 30 mm, preferably from 2.5 to 12 mm. A person skilled in the artknows how to adapt the manufacturing process, in particular as regardsthe 3rd stage (choice of the radical initiator and of the temperatureprogramme), according to the thickness of the acrylic sheet.

As regards the MMA homo- or copolymer, the latter is formed during the4th stage of the process according to the invention and during the 2ndstage according to the alternative form. It is predominantly composed ofMMA.

As regards the cast sheet, the latter comprises an MMA home- orcopolymer which constitutes the matrix in which the block copolymerB(-A)_(n) is homogeneously dispersed. The block copolymer has a tendencyto settle process described is less sensitive to the temperature than acast sheet reinforced using particles of core-shell type.

Use of the Cast Sheets According to the Invention

The sheets manufactured can be used in the manufacture of window panes(in particular in car windows, such as front, rear, side or transparentroof windows), soundproof walls, flat screens, billboard or displaydevices, and the like, or else can be converted to various articles bythermoforming, cutting out, polishing, adhesive bonding or folding.These sheets can be used in particular to manufacture bathroom fittings(baths, sinks, shower trays, and the like). For this, the sheets arethermoformed in a way known to a person skilled in the art.

In the case of car windows, the sheets can advantageously comprise atleast one additive having the role of reflecting infrared radiationand/or one additive having the role of blocking UV radiation.

EXAMPLES Atomic force microscopy

The morphologies of the sheets were observed microscopically by atomicmicroscopy (in tapping mode) after surface improvement with a diamondknife and at a temperature of −60° C. Only the images in phase mode,sensitive to the viscoelastic properties of the material, are given.

Gel Permeation Chromatography (GPC)

The molecular masses (M_(n): number-average, M_(w): weight-average) weredetermined using gel permeation chromatography with respect to a PMMAstandard.

Other Measurements

The impact strength was determined according to Standard EN 179-2 eU(unnotched Charpy impact, total energy of 2 J and impact speed of 2.9m·s⁻¹). The impact strength is expressed in kJ/m².

The optical characteristics were determined using a DatacolorSpectraflash calorimeter. The Vicat temperatures were obtained using aCeast device according to Standard ISO 306.

Example 1 Manufacture of a Strengthened Cast Sheet 1st Stage:Preparation of a Central Block B Based on Butyl Acrylate and on Styrene

The following are introduced into a 15 litre metal reactor equipped witha double-helix agitator, with a jacket for heating by circulation ofoil, and with a vacuum/nitrogen line: 6880 g of butyl acrylate, 1120 gof styrene, 55 g of dialkoxyamine DIAMS (with a purity of 82% and with acontent of free SG1 of 0.48%), i.e. 45 g of pure DIAMS, 1.6 g of SG1with a purity of 85% (i.e., 1.4 g of pure SG1), which represents a 5 mol% excess per alkoxy functional group carried by the DIAMS, taking intoaccount the 0.48% of free SG1 already present in the DIAMS.

After the introduction of the reactants, the reaction mixture isdegassed three times using a vacuum/nitrogen cycle. The reactor is thenclosed and then stirring (50 rev/min) and heating (set temperature: 125°C.) are begun. The temperature of the reaction mixture reaches 115° C.in approximately 30 min. The pressure stabilises at approximately 1.45bar. The temperature of the reactor is kept stationary at 115° C. for250 min. After cooling, 8008 g of a mixture with a solids content of71%, that is to say a 71% solution of butyl acrylate/styrene copolymerin the excess butyl acrylate, are recovered. The butyl acrylate/styreneratio by weight of the central block B obtained is 80.3/19.7. Analysisof the central block B by size exclusion chromatography gives thefollowing results: M_(n): 69 750 g/mol; M_(w): 124 830 g/mol;polydispersity: 1.8.

2nd Stage: Preparation of the B(-A)_(n) copolymer from the PrecedingCentral Block B

The following are introduced into a 15 litre metal reactor equipped withan agitator comprising two counter-rotating helices, with a jacket forheating with circulation of oil, and with a vacuum/nitrogen line: 493 gof the 71% solution from the 1st stage, 4504 g of unstabilized MMA(MMA), 5 g of a 5% solution of γ-terpinene in MMA (i.e. 50 ppm ofγ-terpinene with respect to the mixture).

After introduction of the reactants, the reaction mixture is degassedthree times under vacuum/nitrogen. The reactor is then closed and thenstirring (50 rev/min) and heating (set temperature: 90° C.) are begun.The temperature of the reaction mixture reaches 85° C. in approximately30 min. The duration of the test is counted started from this point. Thepressure stabilises at approximately 1.6 bar. The temperature of thereactor is kept stationary at approximately 90° C. for 25 min. 4950 g ofan 18% syrup of block copolymer are recovered. The ratio by weight ofthe PMMA branches with respect to the central block of butylacrylate-styrene copolymer is 1.6.

3rd Stage: Preparation of a PMMA Sheet with a Thickness of 4 mm from theSyrup of the 2nd Stage

The following are poured into a conical vacuum flask: 80 g of the syrupobtained in the 2nd stage, 85 g of unstabilized MMA, 3.0 g of a 5%solution of azobisisobutyronitrile in MMA (i.e. a content of AIBN of 900ppm with respect to the mixture), 1.7 g of a 1% solution of γ-terpinenein MMA (i.e. a content of γ-terpinene of 100 ppm with respect to themixture).

The constituents are mixed and then carefully degassed under vacuum forat least 20 min. The mixture is then poured into a mould with dimensionsof 200×200×4 mm composed of two glass sheets equipped with an elastomerseal. The mould is then introduced into an oven with programmable aircirculation. Polymerization is carried out for 6 hours at a temperatureof 70° C.

After cooling, an impact-strengthened PMMA sheet is recovered which hasthe following characteristics: thickness: 4 mm, easy removal from themould, the surface is smooth and glossy. The content of central block Bin the sheet: 3.4% by weight. Its GPC characteristics are: M_(w): 1 112115 g/mol; poly-dispersity: 3.2.

The sheet is transparent and show no yellowing. The calorimetriccharacteristics in transmission are as follows: L*: 96.81/a*: 0.06/b*:0.13, Haze: 0.87, Yellow index YIE 313: 0.29, White index WIE 313: 91.4,Transmission at 550 nm: 91.98, impact strength: 28.4±0.8 kJ/m² Vicattemperature: 108.5° C.

Example 2 Preparation of Strengthened Cast Sheets Comprising SeveralContents of Central Block B 1st Stage: Preparation of a Central Block BBased on Butyl Acrylate and on Styrene

The following are introduced into a 2 litre metal reactor equipped witha helical agitator, with a jacket for heating by the circulation of oil,and with a vacuum/nitrogen line: 616 g of butyl acrylate, 84 g ofstyrene, 2.4 g of dialkoxyamine DIAMS (with a purity of 94% and with afree SG1 content of 0.35%), i.e. 2.3 g of pure DIAMS, 0.09 g of SG1 witha purity of 85% (i.e. 0.077 g of pure SG1), which represents a 5 mol %excess per alkoxy functional group carried by the DIAMS, taking intoaccount the 0.35% of free SG1 already present in the DIAMS.

After introduction of the reactants, the reaction mixture is degassedthree times under vacuum/nitrogen. The reactor is then closed and thenstirring (50 rev/min) and heating (set temperature: 125° C.) are begun.The temperature of the reaction mixture reaches 113° C. in approximately30 min. The pressure stabilises at approximately 1.5 bar. Thetemperature of the reactor is kept stationary at 115° C. for 522 min.After cooling, 608 g of a mixture with a solids content of 67% arerecovered. The excess butyl acrylate is subsequently removed byevaporation at 70° C. under reduced pressure for 3 h and replaced by 700g of MMA. 1110 g of a 37% solution in MMA of a stripped macroradical(freed from its excess butyl acrylate) are thus recovered. The butylacrylate/styrene ratio by weight of the macroradical obtained is 83:17.GPC analysis of the central block B gives the following results: M_(n):96 430 g/mol; M_(w): 201 000 g/mol; polydispersity: 2.1.

2nd Stage: Preparation of a Syrup Comprising a Triblock Prepared fromthe Preceding Macroradical.

The following are introduced into the reactor already used for stage 1:132 g of the 37% solution in MMA of the macroradical from stage 1, 566 gof unstabilized MMA, 0.7 g of a 5% solution of γ-terpinene in MMA (50ppm of γ-terpinene).

After introduction of the reactants, the reaction mixture is degassedthree times under vacuum/nitrogen. The reactor is then closed and thenstirring (50 rev/min) and heating are begun. The temperature of thereaction mixture reaches 85° C. in approximately 15 min. The duration ofthe test is counted starting from this point. The pressure stabilises atapproximately 1.5 bar. The temperature of the reaction mixture is keptstationary at approximately 90° C. for 55 min. At the end of the test,after cooling, 620 g of a 15.6% syrup of block copolymer are recovered.Branches/block B ratio by weight of the copolymer obtained: 1.22.

3rd Stage: Preparation of a Series of Impact-Strengthened Sheets with aThickness of 4 mm from the syrup of example 2.

Sheets are manufactured from the mixtures which are described in TableI. The mixtures are stirred, degassed under vacuum and then poured intomoulds according to a procedure identical to that described in Example1.

TABLE 1 Sheet 6 Sheet 2 Sheet 3 Sheet 4 Sheet 5 compar- inventioninvention invention invention ative Syrup from 99.9 89.9 69.9 49.9 0stage 2 [%] AIBN [ppm] 900 900 900 900 900 γ-Terpinene 100 100 100 100100 [ppm] Unstabilized 0 10 30 50 100 MMA (%)

The moulds are then introduced in a horizontal position into an ovenwith programmable air circulation. Polymerization is carried out at 70°C. for 7 hours. PMMA sheets which can be easily removed from the mouldsare recovered and have the following characteristics:

TABLE II Content of Impact block B in Haze Vicat Residual strength Sheetthe sheet [%] [%] [° C.] MMA [%] [kJ/m²] 2 (inv.) 6.3 9.8 29.5 3 (inv.)6.1 3.7 110.7 1.2 31.0 4 (inv.) 4.8 2.9 1.1 41.8 5 (inv.) 3.4 2.5 108.51 41.3 6 (comp.) 0 1 116.0 <1 12.0

It is observed that the impact strength of the sheets does not increaseuniformly with the content of central blocks in the sheet. The strengthis maximal for a content of central block of between 3.4 and 4.8% for atransparency which is very good (haze of 2.5%). The low content ofresidual MMA will also be noted.

Sheets 2 and 4 were studied using an atomic force microscope (AFM). FIG.1 represents an AFM image of sheet 2. FIGS. 2 and 3 represent AFM imagesof sheet 4. For both sheets, the presence of uniformly distributedsubstantially spherical particles is noted in the images. The particlesare nodules within which subnodules are present. The particles of sheet2 have a size of the order of 300-500 nm. Those of sheet 4 have a sizeof the order of 100-200 nm.

Influence of the Temperature on the Transparency

The transparency (haze H in %) as a function of the temperature T (in °C.) was also measured for the cast sheet 5 (curve 1) and for a castsheet obtained from a commercial impact modifier, the grade ZK 6BR soldby Roehm Gmbh (curve 2).

It is found that the haze is not greatly influenced by the temperatureover the curve 1, but the influence is much more substantial for thecurve 2.

1. Process for the preparation of impact-strengthened cast PMMA sheetscomprising the following stages: 1) heating a mixture comprising: atleast one alkoxyamine Z(-T)_(n), in which Z denotes a polyvalent groupand n is an integer greater than 2, and the monomer(s) intended to forma central block B  to a temperature sufficient to activate thealkoxyamine and polymerize the monomer(s); 2) reactivating the centralblock B, optionally mixed with the unconsumed monomer(s) from stage 1,in the presence of the monomer(s) intended to form the branches A; 3)adding methylmethacrylate (MMA) and optionally at least one comonomer Mwhich can be copolymerized by the radical route with methyl methacrylate(MMA) and at least one radical initiator to the mixture obtained instage 2; 4) casting the mixture from stage 3 in a mould and thenheating.
 2. Process according to claim 1, in which the central block Bexhibits an overall glass transition temperature of less than 0° C. 3.Process according to claim 1, in which the central block B is preparedfrom a mixture comprising at least one monomer selected from the groupconsisting of: acrylic monomers of formula CH₂═CH—C(═O)—O—R₁ where R₁denotes a hydrogen atom or a linear, cyclic or branched C₁-C₄₀ alkylgroup optionally substituted by a halogen atom or a hydroxyl, alkoxy,cyano, amino or epoxy group, methacrylic monomers of formulaCH₂═C(CH₃)—C(═O)—O—R₂ where R₂ denotes a hydrogen atom or a linear,cyclic or branched C₁-C₄₀ alkyl group optionally substituted by ahalogen atom or a hydroxyl, alkoxy, cyano, amino or epoxy group;vinylaromatic monomers.
 4. Process according to claim 1, in which eachbranch A exhibits an overall glass transition temperature of greaterthan 0° C.
 5. Process according to claim 1, in which each branch A isprepared from a mixture comprising MMA and optionally at least onemonomer chosen from: acrylic monomers of formula CH₂═CH—C(═O)—O—R₁,where R₁ denotes a hydrogen atom or a linear, cyclic or branched C₁-C₄₀alkyl group optionally substituted by a halogen atom or a hydroxyl,alkoxy, cyano, amino or epoxy group methacrylic monomers of formulaCH₂═C(CH₃)—C(═O)—O—R₂ where R₂ denotes a hydrogen atom or a linear,cyclic or branched C₂-C₄₀ alkyl group optionally substituted by ahalogen atom or a hydroxyl, alkoxy, cyano, amino or epoxy group;vinylaromatic monomers.
 6. Process according to claim 1, in which thegroup Z is chosen from the groups:

in which R₃ and R₄, which are identical or different, represent a linearor branched alkyl radical having a number of carbon atoms ranging from 1to 10, phenyl or thienyl radicals optionally substituted by a halogenatom, or else by a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 4 or else by nitro, alkoxy, aryloxy,carbonyl or carboxyl radicals; a benzyl radical, a cycloalkyl radicalhaving a number of carbon atoms ranging from 3 to 12, a radicalcomprising one or more unsaturations; B represents a linear or branchedalkylene radical having a number of carbon atoms ranging from 1 to 20; mis an integer ranging from 1 to 10;

in which R₅ and R₆, which are identical or different, represent aryl,pyridyl, furyl or thienyl radicals optionally substituted by a halogenatom, or else by a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 4 or else by nitro, alkoxy, aryloxy,carbonyl or carboxyl radicals; D represents a linear or branchedalkylene radical having a number of carbon atoms ranging from 1 to 6, aphenylene radical, a cycloalkylene radical; p being an integer rangingfrom 1 to 10;

in which R₇, R₈ and R₉, which are identical or different, have the samemeanings as R₃ and R₄ of the formula (I) and q, r and s are integersranging from 1 to 10;

in which R₁₀ has the same meaning as R₅ and R₆ of the formula (II), t isan integer ranging from 1 to 4 and u is an integer of between 2 and 6(the aromatic group is substituted);

in which R₁₁, has the same meaning as the R₁₀ radical of the formula(IV) and v is an integer between 2 and 6;

in which R₁₂, R₁₃ and R₁₄, which are identical or different, represent aphenyl radical optionally substituted by a halogen atom, such as Cl orBr, or else by a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 10, W represents an oxygen, sulphur orselenium atom and w is equal to zero or 1;

in which R₁₅ has the same meaning as R₃ of the formula (I) and R₁₆ hasthe same meaning as R₅ or R₆ of the formula (II);

in which R₁₇ and R₁₈, which are identical or different, represent ahydrogen atom, a linear or branched alkyl radical having a number ofcarbon atoms ranging from 1 to 10, an aryl radical, optionallysubstituted by a halogen atom or a heteroatom.
 7. Process according toclaim 1, in which the nitroxide T is represented by the structure:

in which R₁₉, R₂₀, R₂₁, R₂₂, R₂₄ and R₂₄ denote: linear or branchedC₁-C₂₀, preferably C₁-C₁₀, alkyl groups, which may or may not besubstituted, substituted or unsubstituted C₆-C₃₀ aryl groups, saturatedC₁-C₃₀ Cyclic groups, and in which the R₁₉ and R₂₂ groups can form partof an optionally substituted cyclic structure R₁₉—CNC—R₂₂ which can bechosen from:

in which x denotes an integer between 1 and
 12. 8. Process according toclaim 7, in which the nitroxide T has the formula

R_(a) and R_(b) denoting identical or different alkyl groups having from1 to 40 carbon atoms which are optionally connected to one another so asto form a ring and which are optionally substituted by hydroxyl, alkoxyor amino groups and R_(L) denoting a monovalent group with a molar massof greater than 16 g/mol.
 9. Process according to claim 1, in which thenitroxide T has the formula:


10. Process for the preparation of impact-strengthened cast PMMA sheetscomprising the following stages: 1) forming a mixture is comprising: a)a block copolymer B(-A)_(n) composed of n branches A, wherein A exhibitsan overall glass transition temperature of greater than 0° C. andwherein each branch A is prepared from a mixture comprising MMA andoptionally at least one monomer chosen from: (1) acrylic monomers offormula CH₂═CH—C(═O)—O—R₁ where R₁ denotes a hydrogen atom or a linear,cyclic or branched C₁-C₄₀ alkyl group optionally substituted by ahalogen atom or a hydroxyl, alkoxy, cyano, amino or epoxy group; (2)methacrylic monomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ where R₂ denotes ahydrogen atom or a linear, cyclic or branched C₂-C₄₀ alkyl groupoptionally substituted by a halogen atom or a hydroxyl, alkoxy, cyano,amino or epoxy group (3) vinylaromatic monomers and connected by theircovalent bonds to a central block B wherein central block B exhibits anoverall glass transition temperature of less than 0° C., and whereinblock B is prepared from a monomer mixture comprising at least onemonomer selected from the group consisting of: (a) acrylic monomers offormula CH₂═CH—C(═O)—O—R₁, where R₁ denotes a hydrogen atom or a linear,cyclic or branched C₁-C₄₀ alkyl group optionally substituted by ahalogen atom or a hydroxyl, alkoxy, cyano, amino or epoxy group, (b)methacrylic monomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ where R₂ denotes ahydrogen atom or a linear cyclic or branched C₁₋₄₀ alkyl groupoptionally substituted by a halogen atom or a hydroxyl, alkoxy, cyano,amino or epoxy group, (c) vinylaromatic monomers, n denoting an integerof greater than or equal to 2, preferably between 2 and 10 andadvantageously between 2 and 8 b) methyl methacryllate (MMA) and c)optionally at least one comonomer M and at least one radical initiator;2) casting the mixture obtained in stage 1 in a mould and then it isheated in order to obtain a cast sheet.
 11. Process according to claim1, in which the proportion of central block B in the sheet is by weightbetween 2 and
 5. 12. Sheet obtained according to the process of claim 1.13. The sheet of claim 12 comprising window panes, soundproof walls,flat screens, billboards or display devices.
 14. The sheet according toclaim 16, wherein said homo- or copolymer is a MMA homo- or copolymer inwhich particles of block copolymer B(-A)_(n), existing in the form ofsubstantially spherical nodules inside of which one or more subnodule(s)having the same composition as the methyl methacrylate homo- orcopolymer are present, are homogeneously dispersed.
 15. The sheetaccording to claim 14, in which the size of the particles is of theorder of 100-600 nn.
 16. The sheet according to claim 12, wherein saidsheet is cast sheet comprising a homo- or copolymer.
 17. The process ofaccording to claim 1 wherein in said alkoxyamine Z(-T)_(n), Z denotes apolyvalent group and n is an integer of between 2 and
 10. 18. Theprocess of claim 3, wherein: a) said acrylic monomers of formulaCH₂═CH—C(═O)—O—R₁, are selected from acrylic acid; methyl, ethyl,propyl, n-butyl, isobutyl, tert-butyl, 2-ethylhexyl or glycidylacrylate; hydroxyalkyl acrylates; or acrylonitrile; b) said methacrylicmonomers of formula CH₂═C(CH₃)—C(═O)—O—R₂ are selected from methacrylicacid; methyl, ethyl, propyl, n-butyl, isobutyl, tert-butyl, 2-ethylhexylor glycidyl methacrylate; hydroxyalkyl methacrylates; ormethacrylonitrile; and c) said vinylaromatic monomers, are selectedfrom, styrene, substituted styrenes, α-methylstyrene, monochlorostyreneor tert-butylstyrene.
 19. Process according to claim 11, in which theproportion of central block B in the sheet is by weight between 2.5 and4.5%.
 20. MMA homo- or copolymer according to claim 15, in which thesize of the particles is of the order of 100-250 nm.